To date, the Redundant Arrays of Inexpensive disks (RAID) techniques have been provided in order to operate a plurality of storage devices in combination as a virtual disk. Also, as one of the storage devices that are applicable to the RAID technique, a storage device using a flash memory as a storage medium is provided. In a flash memory, an oxide film that becomes an insulator for each time of writing is deteriorated by penetrating electrons, and thus the flash memory has a number of writable times, which is an upper limit number of writable times. Thus, a storage device using a flash memory as a storage medium has information on the total bytes written (TBW), which indicates the amount of writable data based on the number of writable times.
As a related art technique, for example, a technique is provided in which when data is written into a RAID configuration, the data of one SSD selected from a plurality of solid state drives (SSDs) having a larger number of write times in the RAID configuration than a threshold value is written into a spare disk, and the RAID configuration is reorganized using the spare disk. Also, a technique is provided in which data stored in a second storage medium is copied to a third storage medium such that the difference between the number of write times of a first storage medium and the number of write times of the third storage medium after copying the data in the second storage medium to the third storage medium is larger than the difference between the number of write times of the first storage medium and the number of write times of the second storage medium. As related-art technical literature, Japanese Laid-open Patent Publication Nos. 2013-206151 and 2013-041394 are disclosed.